Why does my straight piped chevy pop when I rev it up?

[Devin]

I have a ole 93 chevy 3/4 ton 350 TBI and I took the cats off and mufflers off and replaced it with 3 inch pipe all the way out the back with two 4 inch tips on each side of the truck. Its not too loud it actually sounds pretty nice kind of like a flow master. But my concern is I was reving it up and I got up to about 3500 RPMs and let off the gas and a big fireball came out the tips. My timing is a little advanced to about 12 degrees. Should I reset it to factory timing? It does burn rich. What cause it to pop like that? And yes I know it is illegal to remove the cats and mufflers.....
It also does it when I down shift. Im pretty sure it too lean, how do I fix it?
The timing is supposed to be 0 degrees BTDC. I was told to advace it to 12 degrees BTDC to increase HP. Should I go back to stock?
The stock sticker under the hood says 0 degrees BTC in P or N.

 

[Wes]

Re-set your timing to stock and see if that cures the problem before you scratch your head trying to figure out what is wrong. Remember your engine was designed (factory tuned) to work with back pressure.....like you get with exhausts.

 

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Exhaust is too big causing it to run rich . At least put a resonator back on .